In the photolithography technology used in the semiconductor manufacturing process, shortening of the wavelength of the exposure light source has been progressed as means for increasing the resolution. To date, the exposure light source has shifted from g-line (436 nm) and i-line (365 nm) by mercury lamp to KrF excimer laser (248 nm) and ArF excimer laser (193 nm), and furthermore, the use of EUV (Extreme Ultra Violet) light with a dominant wavelength of 13.5 nm is also being studied.
In a photolithography step of a semiconductor or a liquid crystal display manufacturing process, a pattern is formed by irradiating light to a semiconductor wafer or an original plate for liquid crystal coated with resist; if, however, a foreign matter is adhered to the lithography mask or the reticle used on this occasion (hereinafter collectively referred to as “exposure original plate”), this foreign matter absorbs light or bends light, whereby the transferred pattern becomes deformed or the edges become coarse, and the base becomes dirty black with the resulting problems of impaired size, quality, appearance and the like.
Though these processes are usually performed in a clean room, it is still difficult to keep the exposure original plate clean at all times. Therefore, in general, a means called pellicle for fending off foreign matters is installed on the exposure original plate before the exposure is conducted.
This pellicle is generally composed of a rectangular pellicle frame, a pellicle film attached in a slack-free manner to an upper end face of the pellicle frame, an airtight gasket formed on a lower end face of the pellicle frame, as well as other minor parts, and the said pellicle film exhibits a high transmittance with respect to exposure light. As the said airtight gasket, an adhesive or the like is used.
If such a pellicle is installed on the exposure original plate, the foreign matters do not directly adhere to the exposure original plate for they are intercepted by the pellicle. Then, in the photolithography step, if the focal point is aligned with the pattern of the exposure original plate, the foreign matters on the pellicle are off the focal point and hence their images are not transferred, and thus the problems such as deformation of the pattern can be solved.
Incidentally, the material for the pellicle film is to have a high transmittance and selected in accordance with the type of exposure light. For example, when exposure light is g-line (436 nm), i-line (365 nm), KrF excimer laser (248 nm), or ArF excimer laser (193 nm), then nitrocellulose, cellulose acetate, fluorine-based polymer and the like are used as the pellicle film.
On the other hand, in the newly developed EUV exposure, the transmittance of the conventional pellicle films are so low that they are difficult to perform the role. Therefore, as a pellicle film to be used for EUV exposure, ultrathin silicon or graphene, which show high transmittance for EUV light, are being studied.